Unapproved Mod

Hypoxia results from hacking a built-in oxygen system with a pulse-demand regulator.


Almost any airplane more than a handful of years out of the factory has been modified to some extent. These pages have, on several occasions, discussed the difference between minor modifications requiring only a logbook entry, major mods accomplished as a field approval with an FAA Form 337 and more complicated additions necessitating a supplemental type certificate (STC). For my own airplane, which has been in service coming up on 50 years, I keep a three-ring binder documenting and preserving the Forms 337 and STC paperwork. The binder isnt full, but it wont take much before Ill need to find another place to keep all those important forms.

While most modifications are seemingly benign-new avionics, auxiliary fuel tanks, better instrumentation-complications can arise when we start modifying the modifications. For example, some autopilots arent compatible with tip tanks, and some modern landing light technology wont fit in a cowling also containing a turbo normalizing system. The law of unintended consequences clearly is at work here. But the important takeaway is there is no way to reliably predict what those consequences may be when combining various modifications. Thats one reason STCs can be so expensive-the FAA often requires relatively extensive and expensive testing to ensure complications are minimized.

The point is altering equipment the aircraft came with usually isnt a good idea unless someone has looked in detail at the consequences. Thats true for engines, avionics, airframes and various accessories added at the factory or somewhere else in the aircrafts life. Its especially a bad idea when, for example, modifications are made in the face of express warnings against them, and incorrect components are used with the modified system.


On July 30, 2009, at 2149 Eastern time, a Cirrus SR-22 was destroyed when it impacted terrain in Ravenswood, W.V. The solo airline transport pilot sustained fatal injuries. Instrument conditions prevailed and the flight was operating on an IFR flight plan. It originated at York Municipal Airport (JYR), York, Neb., about 1840, and was bound for Eagle Creek Airpark (EYE), Indianapolis, Ind.

According to the airplanes owner-who did not hold a pilot certificate-he hired the accident pilot to fly him between JYR and EYE for business during the week. On the day of the accident, he and the accident pilot completed the flight from EYE to JYR at 6000 feet, and the pilot then departed on the return flight to EYE.

The airplane was incrementally cleared to FL250. A data recorder indicated the pilot activated the oxygen system at 1852, at 12,160 feet. At 1905, the controller noted the pilot was “climbing to the wrong altitude” and “stepping all over himself.” At 1925, the pilot requested a descent to 12,000 feet. After several attempts, the controller verified the airplane and issued a clearance to descend to FL240. The pilot acknowledged the clearance but did not begin a descent. The controller remarked that the pilot sounded “in distress and out of breath.”

The last radio transmission received from the airplane, at 1937, was the pilots labored breathing. At 2051, the airplane crossed directly over EYE at 25,000 feet, and maintained its on-course heading. An Ohio Air National Guard aircraft intercepted the accident airplane, but once alongside, its crew reported an “unresponsive individual who appeared to be unconscious.” The intercepting aircraft remained with the airplane until it departed controlled flight and descended into terrain.


The airplane wreckage exhibited a fuel odor and all major components were accounted for at the scene. The airplane was significantly fragmented and scattered over the entire area. Fragments associated with the engine, propeller blades, and flight control system were accounted for along the entire length of the wreckage path.

Oxygen system components recovered at the accident site included an intact 77.1 cubic-foot capacity oxygen tank, cannulas and masks. Oxygen flow was controlled by a switch located in the cockpit, which electronically actuated the regulator. A pulse-demand regulator system had been installed downstream of and operated with the airplanes existing oxygen system by means of an in-line regulator. The pulse-demand system provides a short burst of oxygen when it detects inhalation, rather than a continuous flow of oxygen. The NTSB subsequently concluded the pulse-demand regulators addition to the existing built-in oxygen system was installed without the use of FAA-approved installation data.

An entry in the pilots logbook for a flight dated March 12, 2009, conducted in the accident airplane on the intended route of the accident flight, stated, “EPIC!! Near Payne Stewart repeat at FL250….” A friend of the pilot stated the pilot mentioned the incident, stating he had been nearly incapacitated, and learned to watch more closely for the signs of hypoxia. The pulse-demand regulators operating manual stated face masks employing a dilution bag should not be used. All oxygen masks supplied with the factory-installed oxygen system and aboard the aircraft were equipped with diluter bags.

The manual accompanying the airplanes built-in oxygen system stated, “The use of other breathing equipment in conjunction with the built-in portion of the system has not been tested, nor is it FAA-Approved.”

Probable Cause

The National Transportation Safety Board determined the probable cause of this accident to include, “The pilots improper modification of the certified, on-board oxygen system, which resulted in incapacitation due to hypoxia, and the airplanes subsequent uncontrolled descent into terrain.” In fact, this is a pretty classic case of hypoxia-induced incapacitation. Its also a classic case of unapproved equipment having unintended-though not entirely unanticipated-consequences. Ironically, if the pilot used the pulse-demand regulator with a portable oxygen tank and the appropriate masks-and heeded the warning signs from a previous flight-its likely we wouldnt writing about him.

But our moral this month has less to do with hypoxias dangers than it does using unapproved modifications and components, especially those known to exhibit incompatibilities. And in the scheme of things, a pilot could buy several oxygen tank refills for the price of one pulse-demand regulator.


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